The present invention relates to a technology including an image correcting device configured to create an output image through geometrical correction of an input image.
In recent years, in several application fields including a monitoring camera and video door phone, there is a demand for obtaining images of a wide visual field. As a solution to meet this demand, video cameras with a fisheye lens and the like that obtain images of the range of about 180 degrees in the horizontal angle of view are practically used. However, in an image captured with such a device, in general, a straight line in the space of a subject (for example, a straight line in the real world) is greatly distorted. Thus, there is a demand of performing geometrical correction of the captured image for reduction in the degree of distortion.
For a fisheye lens, depending on the design concept, there are various applicable projection methods including equidistance projection, equisolid angle projection, and orthogonal projection. In any of these projections, as long as the angle of view to be represented by an image having been subjected to correction, in other words, an image after correction, is smaller than 180 degrees, the distortion of a line in an image captured with a fisheye lens (hereinafter, referred to as ‘a fisheye image’) can be completely corrected by applying perspective transformation, based on a design value/values or a given parameter/parameters estimated by a conventional technology and a projection model.
FIG. 13A shows an example of a fisheye image. Further, FIG. 13B shows an example where this fisheye image has been corrected, applying perspective transformation such that the angle of view in the left/right direction is ensured to be wide. As shown in FIG. 13B, as a result of correction through perspective transformation, although the portions to be linear are corrected to be linear, the marginal portions of the image are extremely expanded with an emphasis on perspective, causing a feeling of strangeness of the image. This is a problem caused by a significant difference between the angle of view represented by the corrected image and the angle of view of viewing that image.
JP 2008-311890 A discloses a technology developed to solve this problem. In the technology disclosed by JP 2008-311890 A, using a cylindrical surface that smoothly connects the front face and the left and right faces, which are substantially planes, correction is performed such that a linear subject extending in the top/bottom direction and a linear subject directing toward the optical axis respectively appear linear while an angle of view exceeding 180 degrees in the left/right direction is ensured.